In general, the curl radius, the curl curvature and the like have been used as an index for objectively describing the degree of a curly hair (Non-patent Document 1). Further, the thickness of the hair, elasticity against the tension, bending stress and the like have been used as an index for objectively describing hair properties concerning a feel such as tensile properties, elastic properties, and softness of the hair (Non-patent Document 2).
In general, the curl radius and the curl curvature can be calculated by an actual measurement of the entire curved shape of a single hair. As a method for evaluating a curly hair, there has been known a method for analyzing penetration speed of an organic substance or inorganic salt to the hair (Patent Document 1). Further, there is proposed a method for evaluating the degree of curl of the hair by focusing on a bunch of fibrous tissues constituting cortex cells, and obtaining a ratio of absorbance of amide I (C═O bond) contained in this bunch to that of amide II (N—H bond) on the basis of a cross-sectional image of the hair (Patent Document 2).
Human hairs (scalp hair) are mainly formed by scaly (layered) cuticle cells covering the surface of the hair, fibrous cortex cells mainly forming the interior of the hair, and medulla cells constituting porous medulla existing at the central part of the hair. It is said that, of these cells, the cortex cell existing in the hair has at least two types: a cell similar to a para cortex cell of wool and a cell similar to an ortho cortex cell of wool (Non-patent Document 3).
Non-patent Document 3 describes a relationship between the abundance ratio of these two types of cortex cells and a shape of the hair. More specifically, it categorizes the hair into the Asian hair (Mongoloid), the Caucasian hair (Caucasoid), and the Ethiopian hair (African), and describes that there is a certain tendency between a variation of curl of the hair depending on the races and the abundance ratio of the two types of cortex cells.
Details of Non-patent Document 4 will be described later.